The plasma membrane defines the boundary of the cell. It is a selectively permeable phospholipid bilayer with embedded proteins. It is composed of 50-75% proteins, 21-50% lipids, and 8% carbohydrates. There are three models that describe its structure: the sandwich model depicts outer and inner protein layers with a phospholipid middle layer; the unit membrane model also has a trilaminar structure but with extended fibrous proteins; the fluid mosaic model views the membrane as a mosaic of integral and peripheral proteins existing within the fluid phospholipid bilayer. The plasma membrane functions to protect the cell, provide shape, regulate transport, receive signals, act as a receptor, and facilitate chemical exchange.

1. Shri Shankaracharya Mahavidyalaya, Junwani , Bhilai
Structure & Function of Plasma Membrane
Dr. Sonia Bajaj
(Head of Department)

2. Introduction
cell membrane was discovered by Negalli and crammer 1855.
Term plasma lemma was coined by plower1931.
The plasma membrane, also known as the cell surface membrane or plasma
lemma, defines the boundary of the cell.
It is a phospholipid bilayer with embedded proteins that encloses every
living cell.
The biology by a membrane and plasma membrane so the following
characteristics-
1. It is a selectively permeable.
2. It is dynamic in nature it is flexible.
3. It have variable protein to phospholipid ratio.

3. Chemical Composition of Plasma Membrane:
The membrane is mainly composed –
Proteins-50-75%
Lipids-21-50%
Carbohydrates-8%
Water makes about 29% of total weight.
lipids-consist of cholesterol, phospholipids and galactolipids.
In the membrane the lipid molecules consist of two parts —a head and two
tails.
The head is composed of glycerol and is hydrophilic
where as the tails are composed of fatty acids that are hydrophobic.
The head and tail are usually designated as polar and nonpolar end
respectively.
.

4. Structure of Plasma Membrane
A) Sandwich model (proposed by Danielli and Davson 1935 )-
According to this model-
1. pm is lipoprotein in nature.
2. It is a Trilaminar.
3. Outer and inner layer are made up of tense protein.
4. protein are globular (Alpha protein) and provide stability to the membrane.
5. Middle layer is made up of phospholipids and it transparent.
6. Phospholipids are Amphipathic.
7. Amphipathic-The head is composed of glycerol and is hydrophilic
where as the tails are composed of fatty acids that are hydrophobic.
The head and tail are usually designated as polar and nonpolar.
8. Head are bounded to the protein by H-bonds.
9. The Tail are together by weak Van der waal’s force.

5. Structure of Plasma Membrane
B) Unit membrane model (proposed by David Robertson 1959 )-
1. pm is lipoprotein in nature.
2. It is a Trilaminar.
3. protein are fibrous (Beta -protein) and are extended.
4. Phospholipid structure and arrangement same as that in sandwich model .
5. Average thickness of plasma membrane is 75 -100 A.
6. Both Danielli and Davson and Robertson model are collectively called LAMILLAR
MODELS.

6. Structure of Plasma Membrane
C) Fluid mosaic model (proposed by Singer and Nicholson -1972)
1. According to this model protein are arranged in the phospholipid layer as mosaic
pattern and so the membrane is described as “protein iceberg in a sea of
phospholipid”.
2. Plasma membrane is lipoproteins and trilaminar.
3. Phospholipid are the main component of plasma membrane
4. Phospholipid bilayer is fluidity.
5. Phospholipid molecules show two types of movements-
A) Transition movement- when the phospholipid molecules changed their position
in the same layer .
B) Flip flop movement-when the phospholipid molecule change their position
between two layer .

7. 6. Protein ( Alpha globular) and are two types –
A) Extrinsic or Peripherial Protein-
1. They are on the outer and inner side of the phospholipid bilayer.
2. They are loosely attached to the phospholipid molecules so are the easily
separable.
3. They can move freely in membrane structure.
4. They make 30% of the membrane protein.
B) Intrinsic and integral protein –
1. They are completely embedded in phospholipid bilayer.
2. They are strongly held to phospholipid molecules, so cannot be separated .
3. They make 70% of total MP protein .
4. some at the channel of water soluble substance show also called tunnel protein
or Aquaporin.

8. Modification of Plasma Membrane
1. Micriovilli
2. Junctional Complexes(Inter
digitation)
3. Tight junction
4. Desmosomes
5. Terminal bars
6. Gap junction
7. Plasmodesmata

9. Function of Plasma Membrane
1. Protects the cell by acting as a barrier.
2. Provides external shape to cytoplasm.
3. Regulates the transport of substances in and out of the cell.
4. Receives chemical messengers from other cell.
5. Acts as a receptor.
6. The chemical exchange between cytoplasm and outside materials take place
through it by diffusion , osmosis, pinocytosis and permeability.